There is known a method for applying a polymer coating onto the surface of a solid filler by wetting the surface of the filler, for example, clay, with a solution of a monomer, such as styrene, acrylic acid or its derivatives, and a radical polymerization initiator, whereupon radiation or thermal polymerization is carried out.
However, the above method does not make it possible to produce polyolefin-based polymer coatings.
There is further known a method for applying a polymer coating onto the surface of a solid filler by polymerizing ethylene on a catalyst deposited on the surface of the filler. The catalyst is a complex organometallic catalyst comprising a compound of a transition metal and an organoaluminium. According to this method, a solid filler, for example, cellulose, and a solvent, for example, n-heptane, are placed in a reaction vessel, and azeotropic drying of the mixture is carried out. This is followed by liquid-phase deposition on the surface of the filler of the first component of the catalyst, i.e. a compound of a transition metal, for example, vanadium tetrachloride, for which purpose said first catalyst component is added to the mixture. As a result, some part of said catalyst component is deposited on the surface of the filler, while the rest of this component remains undeposited in the solvent. Ethylene is then introduced into the reaction vessel to reduce the vanadium tetrachloride to vanadium trichloride, the reduction taking place both on the surface of the filler and in the solvent. The second component of the catalyst, i.e. an organoaluminium, for example, triethyl aluminium, and ethylene are then introduced into the reaction vessel, which is followed by polymerization of ethylene both on the catalyst deposited on the surface of the filler and on the catalyst which remains free in the solvent. The polymerization is carried out at a temperature of up to 90.degree. C. and atmospheric pressure. The polymerizate thus produced is washed from the remainder of the catalyst and dried.
The process under review yields a mixture containing the end product, i.e. the filler with the polymer coating applied thereto, and a considerable amount of the ballast product, i.e. the free polymer. The presence of the free polymer in the mixture narrows down the field of application of the method under review and affects the quality of articles manufactured from such a mixture, its adverse effects including a reduced mechanical strength and increased flammability of articles. The presence of the free polymer in the mixture also accounts for an unnecessarily great consumption of the initial monomer and catalyst; for example, the catalyst comsumption amounts to 0.4 to 4 percent of the weight of the filler on conversion to vanadium tetrachloride. Despite all the above disadvantages, it is impossible to reduce the free polymer content in the mixture, to say nothing of a complete removal of the free polymer from the mixture.
The method under review is further disadvantageous in that it is hard to adjust the thickness of the polymer coating on the filler's surface. Furthermore, the polymer applied to the surface of the filler tends to peel off that surface; the greater the thickness of the polymer coating, the more this effect is pronounced.
Finally, the method under review makes it necessary to wash the end product from the remainder of the catalyst.